Dispersion-strengthened aluminium alloy

ABSTRACT

A dispersion-strengthened material is described which comprises aluminium or aluminium alloy containing a substantially uniform dispersion of ceramic particles to confer dispersion strengthening which is inherently stable at high working temperatures, the ceramic particles having a diameter of less than 400 nm, and preferably in the range 10 nm to 100 nm. Suitable ceramic dispersoids include Al2O3, TiO2, Al3C4, ZrO2, Si3N4, SiC, SiO2.

The invention relates to a dispersion-strengthened aluminium alloyexhibiting improved stability of strengthening at elevated temperature,and to a method of manufacture thereof.

Aluminium alloys are widely used as structural materials in weightcritical applications, such as for aircraft construction. Strength iscommonly achieved by alloying additions such as copper, magnesium,lithium or zinc to produce a dispersion of fine precipitates followingsuitable heat treatment. These conventional aluminium alloys havelimited capability for use at elevated temperatures; or long term creepapplication they are generally not used at greater than 150° C. forshorter term applications 200 to 300° C. might be a more realistic limitto the working temperature range. The alloys arc limited in use by thelimited strengthening exhibited at elevated temperature resulting fromthe tendency for precipitates to coarsen significantly as thetemperature is raised. This reduces their effectiveness as strengtheningphases at elevated temperature, and also their effectiveness asstrengthening phases at room temperature after an elevated temperaturetreatment.

Significant developments have been made using rapid solidificationtechniques to introduce alloy elements that do not coarsen significantlyat temperatures in excess of 200° C. Examples of alloying elements mostcommonly used are Fe, V, Si . Ce etc. These approaches produce aluminiumalloys with good strength at temperatures of up to 400° C. However, theyare difficult to fabricate because at temperatures exceeding 400° C.,the strengthening precipitates coarsen significantly and hence reducetheir strengthening effectiveness. This means that the temperatures forforming components manufactured from such materials must be limited toless than 400° C. Such constraints can impose significant limitations onthe range of engineering components which can be effectivelymanufactured from these materials. Japanese patent publication number082670075 and U.S. Pat. No. 5,632,827 both describe an aluminiummaterial having ceramic dispersoids, which in both cases are formed byin situ development by precipitation during mechanical alloying and dieformation respectively. EP 0751 228 relates to a titanium aluminiumintermetallic having ceramic dispersoids also formed in situ. However,the size and dispersion of ceramic particles formed in this manner isdifficult to control.

The present invention is directed towards the provision of an aluminiumalloy based on principles of dispersion strengthening which mitigatessome or all of the above problems and in particular which exhibitsenhanced dispersoid stability at elevated temperature.

According to a first aspect of the invention, a dispersion-strengthenedmaterial comprises aluminium or aluminium alloy containing asubstantially uniform dispersion of ceramic particles, characterised inthat the ceramic particles have a diameter of less than 400 mm.

The present invention takes a radically different approach from anyprior art technique based on conventional and rapid solidificationroutes which rely on precipitate dispersions whose thermal stability isthus inherently limited by coarsening since it provides an aluminiumalloy dispersion strengthened with particles which are inherently stableat these working temperatures. The strengthening effect produced thusshows greater stability over time at elevated temperatures than will bepossible in any system based on precipitate dispersions. Particle sizeis preferably less than 100 nm and optimally in the range 10-30 nm.Particles which are finer than this become difficult to distributeevenly; particles which are coarser begin to become less effective asstrengthening dispersoids.

Techniques are known to enhance the elastic modulus of aluminium alloysby addition of a dispersion of ceramic particles, but in this field itis found that to achieve elastic modulus modifications particle sizestypically need to be in the range 3000 to 30,000 nm. The currentinvention is directed to the very different materials problem ofdispersion strengthening and produces a material which exhibits enhanceddispersoid stability at elevated temperature. The particle sizesrequired to solve this problem, at typically less than 400 nm andpreferably less than 100 nm, are thus significantly finer than thoseused to modify elastic modulus, and particles of the size used for thelatter purpose would be too large to provide any substantial dispersionstrengthening effect.

To maximise stability the dispersoids are preferably metal oxides,carbides or nitrides. Without limiting the scope of the invention,examples of dispersoid phases are; A₂O₃, TiO₂, Al₃C₄, ZrO₂, Si₃N₄, SiC,SiO₂. The stability of these phases a fabrication, typically by forging,rolling or extrusion processes at high temperature, often greater than500° C., without significant coarsening of the dispersed particles. Thedispersion may be controlled to include more than one type of ceramicdispersoid particle. Dispersoid particle volume fractions can range from1 to 25 volume per cent, but more preferably in the range 5 to 15 volumepercent.

The dispersion may be controlled to include more than one size ofceramic dispersoid particle within the specified size range; that is tosay to include a first set of ceramic dispersoid particles ofsubstantially similar diameter, and at least one further set ceramicdispersoid particles of substantially similar diameter but ofsubstantially different diameter to the first set. The resultant bimodalor multimodal size distribution enables optimistation of interparticlespacing for a given volume fraction of dispersoid.

A surprising result is found when TiO₂ is used as the dispersoid phase.An alloy containing TiO₂ produces better ductility at room temperatureand especially at elevated temperature than when other types ofdispersoid are used. Another advantage is that the aluminium oraluminium alloys containing this particular dispersoid can be aged byheating to above 500° C. and more preferably to 550° C. It is thoughtthat the TiO₂ reacts to form titanium aluminides when the alloy isheated above 500° C.

It will be readily understood that choice of aluminium alloys is notlimited in this invention, but will instead be determined by the balanceof strength and ductility required by given materials applications. Ingeneral, materials in accordance with the invention can be expected tobe more dilute than conventional systems because the dispersoid, to agreat extent, replaces the need for alloy additions to form conventionalprecipitates. Alloy composition may include, but are not limited to:pure aluminium, solid solution alloys containing magnesium and/orlithium, and conventional alloys containing copper, zinc, manganese,lithium. Alloys of aluminium with lithium and magnesium are especiallyappropriate, preferably comprising 0.1 to 1.7 weight percent lithium and0.1 to 4.0 weight percent magnesium, more preferably 0.1 to 0.75 weightpercent lithium and 0.1 to 2.0 weight percent magnesium, most preferably0.1 to 0.4 weight percent lithium and 0.1 to 1.5 weight percentmagnesium.

Substantial improvements to strength at elevated temperature of 300 to500° C. are achieved with this invention. Such improvements are combinedwith outstanding fatigue performance at elevated temperature and goodresistance to creep. More than 95% strength is retained at roomtemperature after soaking at temperatures close to the alloy solidus.

To ensure that dispersoid is present in sufficient quantity to produce asignificant strengthening effect, the dispersoids are conveniently addedas a separate phase to the matrix using a powder metallurgical route.Thus in a further aspect the invention comprises a method of manufactureof a dispersion-strengthened material comprising the mixing of powderedaluminium or aluminium alloy with ceramic particles having a diameter ofless than 400 nm, the blending of the resultant mixture to produce asubstantially uniform dispersion of ceramic particles, and theconsolidation of the resultant blend to produce a solid material. Amechanical alloying step is preferably included in the process toachieve improved uniformity of ceramic particle dispersion.

Typical compositions of materials in accordance with the invention, andproperties thereof, will now be given by way of example only.

A variety of aluminium alloys were blended and mechanically alloyed withalumina or titanium dioxide particles. Powders were compacted andextruded to form 14 mm diameter bar. Tensile test results in an asextruded condition are detailed in Table 1 for room temperature andelevated temperature.

TABLE 1 Tensile Test Results in the As-Extruded Condition DispersoidVolume %, 0.2% Yield 0.2% Yield 0.2% Yield Aluminium Type and StrengthStrength Strength Alloy average (MPa) at (MPa) at (MPa) at Matrixparticle size 24° C. 300° C. 350° C. Commercial 10% Al₂0₃ 395 216 179Purity 13 nm Commercial 10% Ti0₂ 342 223 186 Purity 23 nm Aluminium None168  56  46 0.3 Li 1 Mg Alloy Aluminium 10% Al₂0₃ 424 174 156 0.3 Li 13nm 1 Mg Alloy Aluminium 10% Ti0₂ 332 179 188 0.3 Li 23 nm 1 Mg AlloyAluminium 7.5% Ti0₂ 296 184 150 0.3 Li 23 nm 176 159 1 Mg AlloyAluminium 12.5% Ti0₂ 359 212 201 0.3 Li 23 nm 381 211 189 1 Mg Alloy 185Aluminium 5% Ti0₂ 327 174 146 0.75 Li 23 nm 2 Mg Alloy Aluminium 15%Al₂0₃ 579 221 0.75 Li 13 nm 2 Mg Alloy

What is claimed is:
 1. A method of manufacture of adispersion-strengthened material comprising the steps of: (a) mixing thepowdered or aluminum alloy matrix with ceramic particles added as aseparate phase to the matrix and having a diameter of less than 30 nmwherein the ceramic particle content is in the range 1 to 25 volumepercent; (b) blending of the resultant mixture to produce an essentiallyuniform dispersion of ceramic particles; and (c) consolidating theresultant blend to produce a solid material.
 2. A method of manufacturein accordance with claim 1 further comprising the step of: mechanicallyalloying the powder mixture to produce an essentially uniform dispersionof ceramic particles.
 3. A method of manufacture in accordance withclaim 1 wherein the ceramic particles have a diameter in the range 10 nmto 30 nm.
 4. A method of manufacture in accordance with claim 1 whereinthe ceramic particle content is in the range 5 to 15 volume percent. 5.A method of manufacture in accordance with claim 1 wherein the ceramicparticles are selected from a group consisting of Al₂O₃, TiO₂, Al₃C₄,ZrO₂, Si₃N₄, SiC, and SiO₂.
 6. A method of manufacture in accordancewith claim 1 wherein the dispersion controlled to include more than oneceramic particles.
 7. A method of manufacture in accordance with claim 1wherein the dispersion is controlled to include a first set of ceramicdispersoid particles of similar diameter, and at least one further setceramic dispersoid particles of similar diameter but of differentdiameter to the first set.
 8. A method of manufacture in accordance withclaim 5 wherein the ceramic particles are TiO₂.
 9. A method ofmanufacture in accordance with claim 8 wherein the solid material is agehardened by heating the material to above 500° C.
 10. A method ofmanufacture in accordance with claim 1 wherein the solid material is analuminum alloy containing lithium and magnesium.
 11. A method ofmanufacture in accordance with claim 10 wherein the solid materialcomprises 0.1 to 1.7 weight percent lithium and 0.1 to 4.0 weightpercent magnesium.
 12. A method of manufacture in accordance with claim11 wherein the solid material comprises 0.1 to 0.75 weight percentlithium and 0.1 to 2.0 weight percent magnesium.
 13. A method ofmanufacture in accordance with claim 12 wherein the solid materialcomprises 0.1 to 0.4 weight percent lithium and 0.1 to 1.5 weightpercent magnesium.